Electronic paging systems incorporating portable, battery operated paging receivers use various methods for reducing the power consumption of the receivers to minimize their overall size and to extend battery life. Many of the existinq systems supply power to the receiver circuits only during intermittent and/or predetermined time periods during which data for the receiver is transmitted. The problem of limiting power consumption and extending battery life is exacerbated in miniature radio receivers incorporated in a wristwatch-pager such as that described in U.S. Pat. No. 4,713,808 to Gaskill et al. The small size and portable nature of the wristband receiver, plus the large number of circuits used compared with the number used in a conventional electronic watch, make reduced power dissipation a primary consideration in the design of the wristwatch-pager.
In the system described in the Gaskill patent the duty cycle of the radio receiver is very short, hence, in order to extend battery life it is desirable that power dissipation in the receiver circuits be virtually zero during the relatively long periods when the receiver is inactive. Further, it is desirable that the circuits involved be insensitive to decreases or fluctuations in power supply voltage caused by battery age, power on/off cycling, the resistance of interconnecting power leads, etc.
A conventional method for switching power to a circuit utilizes a switching device such as a series pass transistor; however, there are several disadvantages associated with such circuits. A series pass device, in the on state, has an associated voltage drop that subtracts from the available supply voltage. Such voltage drop is significant in low-voltage, battery operated apparatus.
Furthermore, the current must be consumed in the controlling element of a power switching device, e.g., the base current required to saturate a switching transistor is greater than the collector current divided by beta. In the case of a low-beta pnp switching device implemented in a typical bipolar integrated circuit process optimized for npn devices, the base current of the switching device can be a significant percentage of the total current, and this current is essentially wasted.
Another disadvantage of using a series switching devices is that the die area required for a switching device that carries the total chip current is significant.
A separate, off-chip switching transistor could be used to improve beta at the cost of an additional part and more board area. An npn switching device could be used to reduce the base current demand, but such a device would require a base voltage above the battery voltage for a low collector-emitter voltage V.sub.CE drop if the switching device were placed between the battery positive terminal and the load. It is noted that if a pass device were placed in the ground return path, its V.sub.CE drop could adversely affect ground-referenced signals.
The object of the present invention to provide an improved on-off switching circuit that maximizes battery life.
Another object of the present invention to eliminate the need for a series switching device and thereby eliminate the voltage drop normally associated with such a device.
Yet another object of the present invention is to provide an on-off switch for electronic circuitry which requires little or no power in the off state.
It is another object of the invention to provide an improved bias supply circuit for a system which has a short duty cycle followed by a relatively long power-off time.
Another object of the instant invention is to provide an improved microcircuit bias supply with on-off switching control that results in virtually zero current drain from the circuits during quiescent periods.